1. Field of the Invention
The invention relates to a sputtering apparatus, and particularly to a sputtering apparatus which is used in a film depositing step in manufacturing a semiconductor integrated circuit or the like.
2. Description of the Related Art
In a thin film deposition using sputtering, and particularly in sputtering used in a film depositing step for manufacturing a highly-integrated semiconductor device, it is strongly recommended to deposit a film at the bottom of a fine hole of a high aspect ratio with an excellent step coverage property, that is, to improve the bottom coverage ratio (the film thickness on the bottom face of the hole with respect to that on the upper face of the hole). As the degree of integration of an integrated circuit becomes higher as 64 Mbits, 256 Mbits, and 1 Gbits, for example, the aspect ratio of a hole (the depth of the hole/the diameter or width of the hole) is increased to 2 to 4. It is recommended to fill a fine hole of such a high aspect ratio with a high bottom coverage ratio.
In order to comply with the requirement, improvements have been made so that a film is deposited while allowing only sputter particles (sputter flux) of a small incident angle to enter a fine hole. One of the improvements is a technique called collimate sputtering.
FIG. 4 is a view schematically illustrating a collimate sputtering apparatus as an example of a conventional sputtering apparatus in which the bottom coverage ratio is improved. In the apparatus shown in FIG. 4, a cathode 2 and a substrate holder 3 are disposed so as to oppose each other in a vacuum vessel 1 having an exhaust system. The cathode 2 comprises a magnet mechanism 4, and a target 5 which is located in front of the magnet mechanism 4. A substrate 30 on which a film is to be deposited is placed on the front face of the substrate holder 3.
A collimator 6 is disposed in a space between the cathode 2 and the substrate holder 3. The collimator 6 has a structure in which a number of small cylindrical members are arranged in a segmental form so that their height directions coincide with a direction perpendicular to the substrate 30 (hereinafter, the direction is referred to as the axial direction), whereby many flow paths for sputter particles are segmentally formed along the axial direction. This structure is often called a "grid-shaped" or "honeycomb" structure.
Sputter particles emitted from the target 5 are distributed in accordance with the cosine law. Therefore, also many sputter particles of a large incident angle enter the collimator 6. However, most of such sputter particles are deposited on the wall faces of the flow paths of the collimator 6, with the result that sputter particles passed through the collimator 6 mainly consist of those of a small emittance angle. Consequently, only sputter particles of a small incident angle impinge on the substrate 30, so that the step coverage property for the bottom of a fine hole formed in the surface of the substrate 30 is improved.
In the collimate sputtering apparatus described above, however, the deposition of sputter particles on the collimator 6 reduces the sectional areas of the flow paths of the collimator 6, with the result that the amount of sputter particles which can pass through the collimator 6 is reduced with the passage of time. Therefore, the deposition rate is gradually lowered.
Recently, an apparatus which is called a low-pressure long-distance sputtering apparatus and in which the distance between a target and a substrate (hereinafter, the distance is referred to as the TS distance) is increased (3 to 6 times that of the conventional apparatus) has been developed as a sputtering apparatus which is free from the above problem and which has a high bottom coverage ratio. FIG. 5 is a view schematically illustrating a low-pressure long-distance sputtering apparatus as another example of a conventional sputtering apparatus.
In the apparatus shown in FIG. 5, in the same manner as that of FIG. 4, a cathode 2 and a substrate holder 3 are disposed so as to oppose each other in a vacuum vessel 1 having an exhaust system, a target 5 is located in front of a magnet mechanism 4, and a substrate 30 is placed on the front face of the substrate holder 3. The TS distance is set to be, for example, about 150 to 360 mm. The pressure of the interior of the vacuum vessel 1 is set to be lower than that in the conventional system or to be about 1 mTorr or less. This pressure reduction is conducted in order that the mean free path of sputter particles is increased and sputter particles are less scattered. Since scattering of sputter particles is reduced in level, many sputter particles can impinge on the substrate 30 in a direction substantially perpendicular to the substrate, thereby enabling the bottom coverage ratio of a fine hole to be improved.
The above-described low-pressure long-distance sputtering is superior to the collimate sputtering as well as the conventional sputtering. In the case where sputtering is conducted on a fine hole of the aspect ratio of 2, for example, the conventional sputtering can attain a bottom coverage ratio of about 5 to 7%, and the collimate sputtering that of about 15 to 20%. In contrast, the low-pressure long-distance sputtering can attain a bottom coverage ratio as high as about 40 to 45% under the conditions that the TS distance is 340 mm and the pressure is 0.3 mTorr.
In this way, the low-pressure long-distance sputtering can attain a high bottom coverage ratio required in a next-generation integrated circuit which is further highly integrated, and is a sputtering process which receives greatest attention. However, there remains a problem which even the low-pressure long-distance sputtering cannot solve. This problem is that, in the coverage of a fine hole existing in a peripheral portion of a substrate, the coverage ratio of the bottom of the hole is not balanced.
This will be described with reference to FIG. 6. FIG. 6 is a view illustrating imbalance of the bottom coverage ratio of a fine hole existing in a peripheral portion of a substrate. As shown in FIG. 6, in a fine hole existing in the vicinity of the center axis of the substrate, a film is deposited so as to be substantially uniform over the whole bottom face of the hole. In a fine hole existing in a peripheral portion remote from the center axis, however, a film is deposited in a larger thickness at a corner portion formed by the bottom face and the side wall which is remoter from the center axis (hereinafter, such a corner portion is referred to as "outer corner portion"), and in a very small thickness at a corner portion formed by the bottom face and the side wall which is closer to the center axis (hereinafter, such a corner portion is referred to as "inner corner portion").
When, in the same manner as the bottom coverage ratio, the coverage ratios at the outer and inner corner portions (hereinafter, such a rate is referred to as "corner coverage ratio") are obtained as values with respect to the thickness of a film on the upper face of the hole, the corner coverage ratios can be respectively expressed as (the film thickness t1 at the outer corner portion/the film thickness T on the upper face of the hole).times.100 (%), and (the film thickness t2 at the inner corner portion/the film thickness T on the upper face of the hole).times.100 (%). When film deposition is conducted on a fine hole of the aspect ratio of 2 by the low-pressure long-distance sputtering under the conditions that the TS distance is 340 mm and the pressure is 0.3 mTorr, the corner coverage ratios listed in Table 1 are obtained.
TABLE 1 Peripheral Vicinity of Portion Center Axis Corner Coverage 50% 45% Ratio at Outer Corner Portion Corner Coverage 25% 45% Ratio at Inner Corner Portion
It is considered that such unbalance of the bottom coverage ratio in a fine hole in a peripheral portion of a substrate is largely affected by the direction of the flight of sputter particles reaching the substrate. Namely, many of sputter particles entering a fine hole in a peripheral portion of a substrate obliquely fly from the side of the center axis of the substrate to the side of the peripheral edge of the substrate. Sputter particles which fly in the opposite direction, i.e., from the side of the peripheral edge of the substrate to the side of the center axis are very small in number. This phenomenon seems to cause the above-mentioned imbalance of the coverage ratio of a fine hole in a peripheral portion.
Such imbalance of the bottom coverage ratio may result in a significant defect in device performance, and hence causes the production yield to be lowered. In an example case where a fine hole is formed as a contact hole (or a through hole) and a sputter film such as that described above is deposited as a barrier film for preventing mutual diffusion of a wiring material and a foundation material from occurring, when the bottom coverage ratio is unbalanced, a phenomenon which impairs electric characteristics, such as a junction leak, occurs at an inner corner portion at which the film thickness is smaller, thereby largely lowering the device performance.
In order to produce a larger number of devices from one substrate and improve the productivity, the size of a substrate tends to be enlarged. When a large substrate is used, the above-mentioned imbalance of the bottom coverage ratio in a hole in a peripheral portion of the substrate produces a more serious problem. In other words, when a substrate which is relatively larger in size than the target is used, sputter particles obliquely flying from the outside to the side of the center axis are not entirely expected to reach the portion of the surface area of the substrate and having a diameter which is larger than that of the target, with the result that the film thickness at the inner corner portion is substantially zero.